FORCES BETWEEN MOLECULES TEMPORARY AND PERMANENT DIPOLES DIPOLES

FORCES BETWEEN MOLECULES: TEMPORARY AND PERMANENT DIPOLES

DIPOLES AND INTERMOLECULAR FORCES

What are dipoles? A dipole is simply a molecule (or part of a molecule) with a positive end a negative end.

There are different types of dipoles • Permanent dipoles • Instantaneous dipoles • Induced dipoles

Instantaneous dipoles The following shows how instantaneous dipoles occur

Electron cloud evenly distributed; no dipole Cl Cl

At some instant, more of the electron cloud happens to be at one end of the molecule than the other, electron density is constantly changing.

At some instant, more of the electron cloud happens to be at one end of the molecule than the other; molecule has a dipole.

An un-polarised Cl 2 molecule finds itself next to an HCl molecule with a permanent dipole Cl Cl H Cl

Electrons get attracted to the positive end of the HCL dipole, inducing a dipole in the Cl 2 molecule Cl Cl H Cl

How a dipole is induced in an atom of Xenon This atom is instantly polarised This atom is not yet polarised, but its electrons are repelled by the dipole next to it…

… so it becomes polarised

DIPOLES AND INTERMOLECULAR FORCES

If a molecular substance contains dipoles, they can attract each other. All intermolecular forces arise from the attractive forces between dipoles. There are three kinds of attraction: • Permanent dipole-permanent dipole • Permanent dipole-induced dipole • Instantaneous dipole-induced dipole

Summary

INSTANTANEOUS DIPOLE-INDUCED DIPOLE ATTRACTIONS • Are forces of attraction which operate between all molecules and between the isolated atoms in Noble gases • Are much weaker than other types of intermolecular forces • Are the result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons

Note: The strength of the forces is related to the number of electrons present and hence to the size of the molecule (or isolated atom)

PERMANENT DIPOLE – PERMANENT DIPOLE ATTRACTIONS • Are additional electrostatic forces of attraction between polar molecules • Are stronger than instantaneous dipole-induced dipole forces for molecules of equivalent size

So… If we have two substances, one polar and one non-polar, whose molecules have the same mass (and so approximately the same number of electrons), the polar substance will have a higher boiling point than the non-polar substance.